Two or more metal pieces that are mutually layered are often bonded by a resistance welding, in which the metal pieces are energized to generate Joule heat to melt the metal pieces while pressing the metal pieces.
A plurality of points have to be welded in order to weld, for instance, a current collecting tab and a current collector of a battery (e.g. a button battery) or terminals of a circuit board. Further, in order to provide a separator of a fuel cell, two or more metal plates are integrally bonded by bonding the metal plates at a plurality of points.
When there are concavity and convexity (e.g. embosses) or steps on a surface of a workpiece in the above welding process, it is desired to reduce unevenness of the weld strength and the time required for the welding step.
In view of the above, a welder and a welding method of an electroconductive member of a battery are disclosed in Japanese Laid-Open Publication No. 2002-035945. As shown in FIG. 13, the welder includes two pressure-side weld electrodes (negative electrodes) 1a and 1b. The pressure-side weld electrodes 1a and 1b are respectively attached to electrode holders 3a and 3b each having welding pressure adjusters 2a and 2b that are capable of independently adjusting the welding pressure. The pressure-side weld electrodes 1a and 1b are connected to one of electrodes of a power source 4.
On the other hand, a stationary weld electrode (positive electrode) 6 is disposed on the other side of a workpiece 5 that is composed of a member 5a and a member 5b. The stationary weld electrode 6 is connected to the other one of the electrode of the power source 4.
With the above arrangement, it is asserted that two points can be simultaneously welded by the pressure-side weld electrodes 1a and 1b and the welding pressures applied on the pressure-side weld electrodes 1a and 1b can be equalized by virtue of the welding pressure adjusters 2a and 2b, so that an even nugget can be provided.
However, in order to weld the workpiece 5 at two points, Japanese Laid-Open Publication 2002-035945 requires that the pressure-side weld electrodes 1a and 1b, the welding pressure adjusters 2a and 2b and the electrode holders 3a and 3b have to be provided as separate components. Thus, the arrangement becomes complex, thus failing to reduce the size of the entire welder.
Further, when the workpiece 5 is welded at three or more points, the pressure-side weld electrodes 1a and 1b cannot simultaneously weld the three or more target points on the workpiece 5. Accordingly, it is difficult to apply an even pressure on each of the welding points.